Nitrification

Nitrification reactor at the Eawag laboratory (photo: Alexandra Hug).

Nitrification is an ideal pretreatment for the production of a highly concentrated fertilizer via evaporation. After urine has been stabilised during nitrification, the dissolved compounds can be recovered by evaporating the water without losing ammonia.

Volatile ammonia

During the storage of urine, urea − the main dissolved compound − is degraded by bacteria to ammonium and bicarbonate. As a result, 90% of the nitrogen in stored urine is ammonium (NH₄⁺/NH₃) and the pH reaches values around 9. In these conditions, the dissolved ammonium tends to volatilise as gaseous ammonia, which causes the repugnant smell of urine. In addition, volatilised ammonium means lower nitrogen concentration in the urine, thus less nitrogen is available for nutrient recovery and fertilizer production.

Stable nitrate

A biological process − nitrification − can stabilise stored urine by oxidising ammonium to nitrate. As a consequence, the stabilised urine does not smell and can be further processed into fertilizer products via evaporation. 

Moving bed biofilm reactor (MBBR): Layers of bacteria are attached to plastic biofilm carriers (photo: Bastian Etter).		Membrane aerated biofilm reactor (MABR): Layers of bacteria are attached to a coiled silicone membrane (photo: Kai M. Udert).

Stabilizing bacteria

In the nitrification process, two bacterial groups are involved: ammonium oxidising bacteria and nitrite oxidising bacteria. Both are very sensitive towards changes in pH and nitrogen concentrations. To achieve higher resilience to changes and a stable nitrate production, biofilm reactors tend to be more suitable (see photos above). 

Nutrient product

At the Eawag main building, a process combining nitrification and evaporation transforms the collected urine into a complete nutrient product.

Further readings:

• Brochure: Nutrient recovery from Urine [English] - [Français] - [Deutsch]
  
• Udert, K.M., Wächter, M. (2012) [pdf] Complete nutrient recovery from source-separated urine by nitrification and distillation. Water Research 46(2), 453-464.
  
• Udert, K.M., Larsen T.A., and Gujer W. (2005) Chemical nitrite oxidation in acid solution as a consequence of microbial ammonium oxidation. Environmental Science and Technology 39(11), 4066-4075.
• Udert K.M., Fux C., Münster M., Larsen T.A., Siegrist H., and Gujer W. (2003) Nitrification and autotrophic denitrification of source-separated urine. Water Science and Technology 48(1), 119-130.